In recent years, in TFT substrates forming LCD devices, TFTs using a semiconductor layer comprised of an oxide semiconductor (hereinafter referred to as the “oxide semiconductor layer”) and having satisfactory characteristics such as high mobility, high reliability, and a low off-state current have been proposed as switching elements of pixels as a minimum unit of an image, instead of conventional TFTs using a semiconductor layer comprised of amorphous silicon.
A typical TFT with a bottom gate structure includes a gate electrode formed on, e.g., a glass substrate, a gate insulating film provided so as to cover the gate electrode, a semiconductor layer provided on the gate insulating film so as to overlap the gate electrode, and source and drain electrodes provided on the gate insulating film so as to overlap the semiconductor layer with a gap between the source and drain electrodes, and a channel region is formed in a portion of the semiconductor layer which is exposed between the source and drain electrodes. The source and drain electrodes preferably uses a stacked structure of, e.g., a titanium layer, an aluminum layer, and a titanium layer (Ti/Al/Ti) in order to reduce resistance as much as possible and to prevent the semiconductor layer and a pixel electrode from mutually changing characteristics.
A channel etch TFT is known as such a TFT with the bottom gate structure. The number of photomasks required to form the channel etch TFT is smaller than that required to form an etch stopper TFT that includes a channel protective film functioning as an etch stopper, because the channel etch TFT does not have the channel protective film. Accordingly, the channel etch TFT is advantageous in terms of manufacturing cost. A channel etch TFT is also disclosed in, e.g., Patent Document 1 as a TFT using an oxide semiconductor layer.